Tethered undersea vehicles are in wide spread use in a variety of scientific and commercial applications. The hazard attendant manned submersibles is avoided and prolonged operations at extreme depths are attainable. One recurring problem is the difficulty of knowing the exact location of the vehicle. A support ship suspending a main cable through several thousand feet of ocean has the difficult task of maintaining a preestablished on-station position. This task is further complicated when ocean currents trail the main cable out in a catenary. The effect of these difficulties is that the tethered vehicle is dragged from an object of interest by its tethering whip cable. One attempt to avoid this problem involved the placing of a directional hydrophone on the support vessel and a noise source on the tethered vehicle. The hydrophone on the support vessel is aligned with the noise source and by this means a "fix" is made on the tethered vehicle. However, the resolution of this system is unacceptable when the vehicle is operating at thousand foot or greater depths. At these depths, the area within which the vehicle's noise source could be located could approach nearly a thousand feet in diameter. Even under the best of conditions, the tethered vehicle could be pulled about without giving the vehicle's operators any forewarning.
Another approach uses several hull mounted hydrophones, a transducer and signal processing equipment aboard the support craft. A transponder on the tethered vehicle is triggered by an acoustic signal from the support craft transducer. The hydrophones record the arrivals of the response pulse and suitable processing equipment provides range and bearing information. The accuracy of this system depends on how accurately the position of the hydrophones is known, on the path the acoustic signals take in water, and on the quality of the signal processing equipment aboard the support ship. All three of these variables are factors when considering the inherent random errors of the system. Also to be considered is the fact that the signal processing equipment must be able to interpret the pitch and roll of the ship so as to simulate a stable platform. A further disadvantage of this system is that a large amount of hardware is required. A small computer and, thus, the resultant system cost and operators, manifestly complicate the location of the tethered vehicle.
An alternative is to locate at least three transducer-transponders on the ocean floor. A receiver on the tethered vehicle receives signals from the transducer-transponders and they are compared and processed by equipment on the surface craft. The accuracy of this system is better than the aforediscussed systems. Yet, there is a considerable effort expended when locating and relocating the transponders and their subsequent recalibrations. Needless to say, this is a very time consuming, costly operation.
There is a continuing need in the state of the art for an uncomplicated system for determining the location of a tethered vehicle operating at extreme depths which reduces the possibility of pulling the vehicle from an area of interest by a main support cable.